The invention generally relates to etching metal using sonication.
In a variety of different circumstances, it may be desirable to selectively etch metal in the formation of a semiconductor device. For example, the etching of metal may be related to the formation of a metal silicide layer (a nickel silicide layer, for example), a layer used to reduce metal-to-semiconductor contact resistances in a semiconductor device.
To form a metal silicide layer, a metal layer (nickel, for example) typically is deposited on a semiconductor structure. In this manner, the deposited metal reacts with exposed silicon of the structure to form the metal silicide layer. Not all of the deposited metal layer typically reacts. In this manner, the regions in which the metal layer does not react form excess or un-reacted metal regions that typically are removed by wet etching. As a more specific example, FIG. 1 depicts a semiconductor structure 9 that represents a particular stage in a process to form a complimentary metal oxide semiconductor (CMOS) transistor. For this example it is assumed that the CMOS transistor is formed on a silicon substrate 12. As shown in FIG. 1, the polysilicon layer 18 resides on top of a gate oxide layer 16, and vertically extending nitride spacers 20 may be located on either side of the polysilicon layer 18.
For purposes of creating a nickel silicide layer, a nickel layer 22 may be blanket deposited over existing layers of the structure 9. As depicted in FIG. 1, the deposited nickel layer 22 extends over portions of the silicon substrate 12 as well as extends over a polysilicon layer 18. The regions in which the nickel layer 22 contacts the silicon substrate 12 form parts of the source and drain of the transistor, and the region in which the nickel layer 22 contacts the polysilicon layer forms part of the gate of the transistor in this example.
Thus, the deposited nickel layer 22 contacts the polysilicon layer 18 and the silicon substrate 12, and in these contacted regions, the nickel layer 22 reacts with the polysilicon layer 18 and the silicon substrate 12 to form the nickel silicide layer that extends into regions 26 of a resulting structure 10 that is depicted in FIG. 2. As a more specific example, a particular nickel silicide region 26a may be associated with a drain of the transistor, another nickel silicide region 26b may be associated with a source of the transistor, and another nickel silicide region 26c may be associated with a gate of the transistor.
The deposited nickel does not react everywhere, leaving regions 24 of excess or unreacted nickel. To remove these regions 24, selective wet etching is used to target the nickel but not other substances (such as nickel silicide, for example) to remove the nickel to form a structure 11 that is depicted in FIG. 3. Thus, after the selective wet etching, the unreacted nickel portions 24 (see FIG. 2) are removed, leaving only the regions 26 of nickel silicide film, as depicted in FIG. 3.
The wet etching typically involves submersing a wafer that contains the structure 10 into a nickel selective etchant, or etching fluid, that typically includes both an acid, such as sulfuric acid, and an oxidant, such as hydrogen peroxide or nitric acid. At room temperature, the use of sulfuric acid by itself to etch the nickel is not sufficient due to the potential energy barrier that prevents the oxidation of the nickel in accordance with the Pourbaix chart for nickel. Therefore, an oxidant typically is introduced into the etching fluid to supply the needed energy to oxidize the nickel into an aqueous derivative and thus, dissolve the nickel.
For certain semiconductor devices, an oxidant in the etching fluid may undesirably oxidize and thus, etch substances that are not meant to be etched. For example, elemental germanium substrates, germanium-doped silicon substrates and germanide films are examples of germanium-based substances that typically are highly susceptible to oxidants that are used in the etching of nickel. The etch rates for these germanium substances may be the same or even higher than the etch rate for nickel in the presence of such an oxidant. Therefore, when germanium-based substances are present, the use of conventional etching fluid to etch nickel may undesirably dissolve significant portions of these germanium-based substances.
Thus, there is a continuing need for a better way to selectively etch metal that is disposed on a semiconductor structure that contains certain semiconductor substrates, films and/or layers.